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Hurricane season refuses to blow over

By Jeff Hecht

The record-breaking North Atlantic hurricane season of 2005 just will not end – and it may get worse yet, say meteorologists as yet another storm has surfaced in the Atlantic and the remnants of Tropical Storm Delta blow towards Africa.

On Monday, Delta blew across the Canary Islands on its way towards Morocco. The US National Hurricane Center predicts its remains may reach northern Mauritania on Tuesday.

But this highly unusual transatlantic crossing does not wrap up this already-record-breaking season, which officially ends Wednesday. On Tuesday meteorologists at the NHC named tropical storm Epsilon, which is the record 26th tropical storm of 2005.

Meanwhile, the centre’s annual re-analysis of the storm season may lead to upgrading of the peak strength of July’s Hurricane Emily to the most intense class, Category 5, says NHC forecaster Stacy Stewart. With Hurricanes Katrina, Rita and Wilma all recorded as Category 5 storms, 2005 already holds the record for three such powerful storms in one season. The total of 13 hurricanes is also a record for the North Atlantic hurricane season.

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Look East

Delta was an unusual storm in an unusual season. Hurricanes typically form between the Cape Verdes Islands off Africa and the Lesser Antilles north of Venezuela, then move west into the Caribbean and Gulf of Mexico.

But Delta formed north of the usual hurricane zone as an “extratropical” storm, with a cooler core and a different circulation pattern than tropical storms which form over warm water.

Pushed south by other weather fronts, Delta became tropical as it moved over warmer waters. Instead of heading west, it turned to the east due to the push of prevailing weather patterns, becoming extratropical again before passing over the Canaries, then merging with a larger frontal system headed for Morocco.

Unprecedented behaviour

Storms that form over warm waters often lose their tropical characters as they move over cooler water. Extratropical storms rarely move south and become tropical like Delta, except late in the hurricane season. In October, Hurricane Vince also formed from an extratropical storm then, like Delta, turned to the east and went on to become the first tropical cyclone ever known to have hit Spain.

This year’s bumper crop of hurricanes was fuelled by warmer than average water temperatures and favourable upper-level wind conditions, particularly over the Gulf of Mexico and the Caribbean. Instead of forming between the Cape Verdes Islands and the Lesser Antilles, in 2005 many storms formed close to land and intensified much faster than usual due to the extra energy from the warm waters.

“Wilma went from a tropical storm to Category 5 in 24 hours. That’s unprecedented!” Stewart told New Scientist. Usually there is a week of warning time to prepare as a storm approaches the coast from far out in the ocean.